Zinc sulfide x-ray phosphors



OCt- 16, 1951 R. L. LoNGlNl 2,571,905

ZINC SULFIDE RAY PHosPHoRs Filed Aug. 28, 1947 (Ph osphor Efficiency) X (Xuy Hbaof Zion Coefficien i.)

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WITNESSES: INVENTOR M RichmdL. Longini.

BY f 2,200. i. V

Patented Oct. 16, 1951 2,571,905 ZINC SULFIDE X-RAY PHosPHoRs Richard L. Longini, Pittsburgh, Pa., assignor to' Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania rApplication August 28, 1947, Serial No.`771,113

4 Claims.

MY invention relates to materials which be-- come uorescent to produce visible light under the impact of X-rays and, in particular, relates to a method of combining different substances to produce a maximum yield of visible radiation for a given X-ray energization.

In the medical and other X-ray arts, it is frequently desirable to make visible the X-ray patterns produced by irradiating various objects which are opaque to visible radiation by streams of X-rays, and screens covered by uorescent material on which the transmitted X-ray pattern is incident have been used for this purpose. However, diiiiculties have been found in that the brightness of the image produced on the fluoresi cent screen is undesirably low, so that it has been necessary to view the screens .invdarkened rooms, and to delay observation until the eyes of the observer have had time to accommodate themselves to the low luminous intensity of the View being observed.

One obiect of my invention is, accordingly, to provide a fluorescent material in which the luminous intensity of the iiuorescent image for a given intensity of X-ray radiation is increased over that possible in screens of the prior art.

Another object of my invention is to produce a material which produces visible light of a highly desirable character and intensity when irradiated with X-rays.

Still another object of my invention is to produce a screen for X-ray purposes in which the intensity of the visible image resultingr from a given amount of incident X-radiation is of a higher order than that found in screens of the prior art.

Still another object of my invention is to pro- Vide a method of admixing fluorescent materials of the prior art with such a proportion of ancillary substances as will produce a maximum yield of visible radiation for a given intensity of X- radiation.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawing in which the single figure is a graph showing the effect on X-ray absorption and luminous intensity of various mole fractions of addition agents incorporated in fluorescent materials of the prior art.

Among the materials which have been used in the production of iiuorescence under the action of X-rays are zinc sulphide, and zinc-cadmiumsulphide. Other materials of somewhat analogous chemical classification are known, the name X-ray phosphors being frequently applied to materials for such purposes.

In making measurements of the luminous, efliciency of such materials, I have foundthat while zinc sulphide produces a much larger yield of Visible radiation than does calcium tungstate for a given absorption of X-ray energy, the luminous intensity of the Zinc sulphide screens is made undesirably low because Zinc sulphide' has a very low absorption coeiiicient. I have, however, found that this diiculty can be corrected by admixing with the zinc sulphide an ancillary material or flux in the form of an alkali halide. Among the alkali halides useful for this purpose are caesium iodide (CSI), caesium bromide ('C'sBr), rubidium iodide (RbI), caesium chloride (CsCl), potassium iodide (KI), sodium iodide (NaI) rubidium bromide (RbBr), and rubidium chloride (RbCl). Such materials have a strong tendency to absorb X-rays and may be referred to as heavy alkali halides in view of their atomic weights, These materials increase the X-ray absorption coefficient of the mixture by amounts having the same order in which I have just enumerated them, the caesium iodide producing the 'greatest relative absorption. A given one of these addition agents appears to increase the relative absorption by an amount linearly proportional to the mole fraction of the addition agent.

It has also been found that such addition agents likewise increase the luminous eiiiciency of the iiuorescent material, provided they are not added in too great amounts; that is to say, they increase the ratio of the energy emitted as visible radiation to the energy of the X-radiation absorbed. There is found to be a maximum luminous eiciency for a certain mole fraction of the addition agent, the efficiency first increasing as small amounts of the addition agent are incorporated with the pure zinc sulphide, then rising to a maximum, and then decreasing ultimately to a lower value than that of the pure zinc sulphide at large `molecular fractions of the addition agent. However, I have found that the amount of addition agent which produces the maximum luminous eiciency is lower than that which produces a maximum ratio of the energy of visible light to the energy of the incident X-rays. It appears that the criterion for a maximum yield of visible radiation when X- rays are used as the energizing agent is proportional to the product of the luminous efficiency by the X-ray absorption coeicient. It is one of the important features of my invention, therefore, that I proportion the amount of the addition agent to the pure phosphor in such a Way as to make the above-mentioned product a maximum.

This is illustrated by the curves in the drawing which represent a typical phosphor. In these the abscissae represent mole fraction of the addition agent and the ordinates in curve I represent luminous eiiiciency of the resulting phosphor. Curve II shows the variation of X-ray absorption and curve III shows the product of the ordinates of curves I and II. The maximum on curve III is seen to occur at a larger mole 'fraction than the maximum i'or curve I.

While `I have described zinc sulphide as the particular fundamental material With which the addition agents have been incorporated, the improved results due to use of such materials may be attained with other fluorescent materials such as zinc-cadmium-sulphide. In particular, the principle of proportioning the addition agent so that the product of the luminous eiciency :for electron energization bythe X-ray absorption coeiiicient is a maximum is applicable to vother mixytures than that which I have specifically described above.

To give a very specific embodiment of the principles of my invention, I have found that a mixture of zinc sulphide with caesium chloride present in an amount constituting of 0.2 mole fraction of the mixture produces a. highly desirable screen for X-ray Work.

It is conventional to provide such X-ray phosphors with what are known as activators which are usually present to the order of 0.01%, and such activators may be used with the mixtures which are described as embodiments of my invention. For example, silver is a suitable activater; the element copper is another which may be used.

I claim as my invention:

1. The red reaction product 'of a mixture of zinc sulphide phosphor with caesium chloride, the latter being present in anamount of 0.2 of a mole fraction of the mixture.

2. An X-ray screen consisting essentially of a support coated with a red reaction product of a substance drawn from the group consisting of zinc sulphide and zinc cadmium sulphide, caesium chloride, and an activator selected from the group consisting of the metals silver and copper, said caesium chloride being present in the amount of 0.2 of a mole fraction of the mixture.

3. The iired reaction product of a substance drawn from the group consisting of zinc sulphide and zinc cadmium sulphide mixed with a substance drawn from the group consisting of caesium iodide, caesium bromide, rubidium iodide, caesium chloride, potassium iodide, sodium iodide, rubidium bromide and rubidium chloride, and an activator selected from the group which consists of the metals silver and copper, said caesium chloride being present in the amount of 0.2 .of a mole fraction of the mixture.

4. The fired reaction product of zinc sulphide mixed with caesium .chloride and an activator selected from the group which consists of the metals silver and copper, said caesium chloride being present in the amount of 0.2 of a mole fraction of the mixture.

RICHARD L. LONGINI.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,310,740 Leavy Feb. 9, 1943 2,435,435 Fonda Feb. 3, 1948 2,435,436 Fonda Feb. 3, 1948 FOREIGN PATENTS Number Country Date 449,392 Great Britain June 19, 1936 

3. THE FIRED REACTION PRODUCT OF A SUBSTANCE DRAWN FROM THE GROUP CONSISTING OF ZINC SULPHIDE AND ZINC CADMIUM SULPHIDE MIXED WITH A SUBSTANCE DRAWN FROM THE GROUP CONSISTING OF CAESIUM IODIDE, CAESIUM BROMIDE, RUBIDIUM IODIDE, CAESIUM, CHLORIDE, POTASSIUM IODIDE, SODIUM IODIDE, RUBIDIUM BROMIDE AND RUBIDIUM CHLORIDE, AND AN ACTIVATOR SELECTED FROM THE GROUP WHICH CONSISTS OF THE METALS SILVER AND COPPER, SAID CAESIUM CHLORIDE BEING PRESENT IN THE AMOUNT OF 0.2 OF A MOLE FRACTION OF THE MIXTURE. 